The present invention relates to the analysis of seismic images of the subsurface.
It is known, especially in oil exploration, to determine the position of oil reservoirs from the results of geophysical measurements carried out from the surface or in well bores. In the technology of reflection seismology technology, these measurements involve emitting a wave into the subsurface and measuring a signal comprising diverse echoes of the wave on geological structures being investigated. These structures are typically surfaces separating distinct materials, faults, etc. Other measurements are carried out from wells. Acoustic waves or electromagnetic radiation are then sent into the subsurface.
The measurements are processed to recreate a model of the subsurface, generally in the form of seismic images or echographic images. These images may be two-dimensional (seismic sections) or three-dimensional (seismic blocks). A seismic image consists of pixels, the intensity of which represents a seismic amplitude depending on local impedance variations.
Geophysicists are used to analyzing such seismic images bearing amplitude information. By visual observation they are able to separate subsurface areas having different characteristics with a view to determining the structure of the subsurface.
Automatic methods exist for extracting structural information from seismic images. In particular, it is known to estimate seismic horizons by computer analysis of the amplitude gradients in the seismic image. The horizons thus estimated are called “synthesized horizons” in contrast with horizons determined by manual checking of the images.
One possible method for estimating horizons in a two-dimensional seismic image consists in investigating, starting from one pixel of the image, the direction along which the local amplitude gradient is minimal. By propagating in this direction, a line representing a synthesized horizon is gradually constructed. If the seismic image is three-dimensional, it is possible to estimate horizons in the form of surfaces transverse to the vertical direction, for example by means of the propagation method described in French patent No. 2 869 693.
Synthesized horizons may be processed to generate accumulation values (or synthesized accumulation values) forming a synthesized image, having the same dimensions as the seismic image, containing structural information on the organization of the subsurface. The accumulation values are, for example, calculated on a computer as follows. Starting from each pixel of the seismic image, a horizon is estimated by a gradient propagation method and a value of one is allocated to each pixel of this estimated horizon and a value of zero is allocated to all the other pixels. The sum of the values (1 or 0) allocated to each pixel of the synthesized image in relation to the horizons coming from the various pixels of the seismic image yields an accumulation value. High accumulation values correspond in particular to areas of the image where different horizons converge, while low values instead represent areas where the physical characteristics are relatively homogeneous.
The images formed by the synthesized accumulation values may be transformed in the way described in U.S. Pat. No. 6,771,800 to carry out a chrono-stratigraphic analysis of the seismic images. The transformation is nonlinear and calculated by integrating accumulation values along vertical lines. It provides conversion from a physical time scale to a pseudo-geological time scale. The transformed image has connected components that can be interpreted as corresponding to geological deposits, demonstrating geological hiatuses between them.
Synthesized images calculated by accumulation along estimated horizons constitute a sort of summary of the structural information and are therefore very useful for getting an idea of the geometry of the subsurface. The information they visualize is more structural than lithological, as they no longer involve the seismic amplitude values but only their continuities and discontinuities. This limits their interest to geophysicists, who often prefer to examine seismic images themselves, despite the difficulty they might have in distinguishing the structure in these.